OpenShift Container Platform

Create a KubeletConfig CR to edit kubelet parameters

You can use a KubeletConfig custom resource (CR) to edit a kubelet parameters without modifing the kubelet configuration directly.

As the fields in the kubeletConfig object are passed directly to the kubelet from upstream Kubernetes, the kubelet validates those values directly. Invalid values in the kubeletConfig object might cause cluster nodes to become unavailable. For valid values, see the Kubernetes documentation.

Consider the following guidance:

Edit an existing KubeletConfig CR to modify existing settings or add new settings, instead of creating a CR for each change. It is recommended that you create a CR only to modify a different machine config pool, or for changes that are intended to be temporary, so that you can revert the changes.
Create one KubeletConfig CR for each machine config pool with all the config changes you want for that pool.
As needed, create multiple KubeletConfig CRs with a limit of 10 per cluster. For the first KubeletConfig CR, the Machine Config Operator (MCO) creates a machine config appended with kubelet. With each subsequent CR, the controller creates another kubelet machine config with a numeric suffix. For example, if you have a kubelet machine config with a -2 suffix, the next kubelet machine config is appended with -3.

If you are applying a kubelet or container runtime config to a custom machine config pool, the custom role in the machineConfigSelector must match the name of the custom machine config pool.

For example, because the following custom machine config pool is named infra, the custom role must also be infra:

apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfigPool
metadata:
  name: infra
spec:
  machineConfigSelector:
    matchExpressions:
      - {key: machineconfiguration.openshift.io/role, operator: In, values: [worker,infra]}
# ...

If you want to delete the machine configs, delete them in reverse order to avoid exceeding the limit. For example, you delete the kubelet-3 machine config before deleting the kubelet-2 machine config.

If you have a machine config with a kubelet-9 suffix, and you create another KubeletConfig CR, a new machine config is not created, even if there are fewer than 10 kubelet machine configs.

The following example command and output show a KubeletConfig CR:

$ oc get kubeletconfig

NAME                      AGE
set-kubelet-config        15m

The following example command and output show a KubeletConfig machine config:

$ oc get mc | grep kubelet

...
99-worker-generated-kubelet-1                  b5c5119de007945b6fe6fb215db3b8e2ceb12511   3.5.0             26m
...

The following procedure is an example to show how to configure the maximum number of pods per node, the maximum PIDs per node, and the maximum container log size size on the worker nodes.

Prerequisites

Obtain the label associated with the static MachineConfigPool CR for the type of node you want to configure. Perform one of the following steps:

View the machine config pool:

$ oc describe machineconfigpool <name>

For example:

$ oc describe machineconfigpool worker

Example output

apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfigPool
metadata:
  creationTimestamp: 2019-02-08T14:52:39Z
  generation: 1
  labels:
    custom-kubelet: set-kubelet-config

The metadata.labels parameter specifies labels you can use with a KubeletConfig CR.

If the label is not present, add a key/value pair:

$ oc label machineconfigpool worker custom-kubelet=set-kubelet-config

Procedure

View the available machine configuration objects that you can select:
```
$ oc get machineconfig
```
By default, the two kubelet-related configs are 01-master-kubelet and 01-worker-kubelet.

Check the current value for the maximum pods per node:

$ oc describe node <node_name>

For example:

$ oc describe node ci-ln-5grqprb-f76d1-ncnqq-worker-a-mdv94

Look for value: pods: <value> in the Allocatable stanza:

Example output

Allocatable:
 attachable-volumes-aws-ebs:  25
 cpu:                         3500m
 hugepages-1Gi:               0
 hugepages-2Mi:               0
 memory:                      15341844Ki
 pods:                        250

Configure the worker nodes as needed:

Create a YAML file similar to the following that contains the kubelet configuration:

Kubelet configurations that target a specific machine config pool also affect any dependent pools. For example, creating a kubelet configuration for the pool containing worker nodes will also apply to any subset pools, including the pool containing infrastructure nodes. To avoid this, you must create a new machine config pool with a selection expression that only includes worker nodes, and have your kubelet configuration target this new pool.

apiVersion: machineconfiguration.openshift.io/v1
kind: KubeletConfig
metadata:
  name: set-kubelet-config
spec:
  machineConfigPoolSelector:
    matchLabels:
      custom-kubelet: set-kubelet-config
  kubeletConfig:
      podPidsLimit: 8192
      containerLogMaxSize: 50Mi
      maxPods: 500

where:

spec.machineConfigPoolSelector.matchLabels

Specifies the label from the machine config pool.

spec.kubeletConfig

Specifies the kubelet configuration. For example:

Use podPidsLimit to set the maximum number of PIDs in any pod.
Use containerLogMaxSize to set the maximum size of the container log file before it is rotated.

Use maxPods to set the maximum pods per node.

The rate at which the kubelet talks to the API server depends on queries per second (QPS) and burst values. The default values, 50 for kubeAPIQPS and 100 for kubeAPIBurst, are sufficient if there are limited pods running on each node. It is recommended to update the kubelet QPS and burst rates if there are enough CPU and memory resources on the node.

apiVersion: machineconfiguration.openshift.io/v1
kind: KubeletConfig
metadata:
  name: set-kubelet-config
spec:
  machineConfigPoolSelector:
    matchLabels:
      custom-kubelet: set-kubelet-config
  kubeletConfig:
    maxPods: <pod_count>
    kubeAPIBurst: <burst_rate>
    kubeAPIQPS: <QPS>

Update the machine config pool for workers with the label:

$ oc label machineconfigpool worker custom-kubelet=set-kubelet-config

Create the KubeletConfig object:
```
$ oc create -f change-maxPods-cr.yaml
```

Verification

Verify that the KubeletConfig object is created:
```
$ oc get kubeletconfig
```
Example output
```
NAME                      AGE
set-kubelet-config        15m
```
Depending on the number of worker nodes in the cluster, wait for the worker nodes to be rebooted one by one. For a cluster with 3 worker nodes, this could take about 10 to 15 minutes.

Verify that the changes are applied to the node:

Check on a worker node that the maxPods value changed:
```
$ oc describe node <node_name>
```

Locate the Allocatable stanza:

 ...
Allocatable:
  attachable-volumes-gce-pd:  127
  cpu:                        3500m
  ephemeral-storage:          123201474766
  hugepages-1Gi:              0
  hugepages-2Mi:              0
  memory:                     14225400Ki
  pods:                       500
 ...

In this example, the pods parameter should report the value you set in the KubeletConfig object.

Verify the change in the KubeletConfig object:

$ oc get kubeletconfigs set-kubelet-config -o yaml

This should show a status of True and type:Success, as shown in the following example:

spec:
  kubeletConfig:
    containerLogMaxSize: 50Mi
    maxPods: 500
    podPidsLimit: 8192
  machineConfigPoolSelector:
    matchLabels:
      custom-kubelet: set-kubelet-config
status:
  conditions:
  - lastTransitionTime: "2021-06-30T17:04:07Z"
    message: Success
    status: "True"
    type: Success